UTL_HTTP ORA-00018 error

I am trying to write a PL/SQL procedure that will loop through a list
of URLs and determine whether or not they actually exist. For each
URL, I'm performing the following sequence of procedure/function calls:
UTL_HTTP.BEGIN_REQUEST, UTL_HTTP.GET_RESPONSE, UTL_HTTP.END_RESPONSE.
After about 675 iterations of the loop, I get the "ORA-00018: maximum
number of sessions exceeded" exception. I'm not using persistent
connection support. The basic loop follows. I would appreciate any
help that you can provide.